The system freezes on start up with the boot logs on the display. X.Org does not start successfully.

Solution

To make Ubuntu start in graphical mode again, we will uninstall the fglrx drivers and use the default open source radeon drivers. This will give you graphical mode back again to further configure your Ubuntu installation (and eventually reinstall fglrx).

For this, restart the computer. In boot manager grub, select the default Ubuntu entry, press keyboard key E to edit that entry, and append an S to the line starting with linux. Pressing F10 will boot into runlevel super-user, that will give you a root shell. (Please note that you will not be able to perform the following steps in a root shell provided by Ubuntu’s Recovery mode.)

In the root shell, uninstall the proprietary fglrx driver

sudo aptitude purge fglrx fglrx-amdcccle fglrx-core fglrx-pxpress

and reboot (without modification of the grub entry this time). The system will now use the default radeon drivers and boot into graphical mode. In graphical mode, you may finally reinstall the fglrx drivers, if wished.